1. Field of the Invention
This invention relates to a scanning electron microscope, and more particularly to a scanning electron microscope suitable for measuring the distance between a first point on a specimen and a second point different therefrom.
2. Description of the Prior Art
As is well known, in the scanning electron microscope, a specimen is scanned in two dimensions by an electron beam, and an information signal characteristic of the specimen as thus obtained therefrom (such as secondary electron signal, reflected electron signal, absorbed electron signal, Auger electron signal, cathode luminescence signal and X-ray signal) is introduced into a cathode-ray tube as a brilliance modulation signal. On the other hand, the screen of the cathode-ray tube is scanned in two dimensions in synchronism with the scanning of the specimen by an electron beam which is generated in the cathode-ray tube. Accordingly, the image of the scanned area of the specimen based on the information signal peculiar to the specimen is displayed on the screen of the cathode-ray tube.
In such a scanning electron microscope, it has recently been regarded as especially important to precisely measure the true distance between two points on the specimen as corresponds to any desired two points on the specimen image displayed on the cathode-ray tube. Techniques which meet this requirement are stated in U.S. Pat. No. 4,039,829 entitled "Stereoscopic Measuring Apparatus" and issued to the same assignee as that of the present application on Aug. 2, 1977 and in the copending application entitled "Scanning Type Electron Microscope," Ser. No. 934,716, filed Aug. 21, 1978. According to these techniques, especially the latter, the distance between any desired two points on the specimen can be accurately measured insofar as the magnification of the specimen image and the inclination angle of the specimen to the electron beam are precisely set. In the strict sense, however, the magnification or the inclination angle is not always set precisely. Even if it has been precisely set once, ordinarily the set value will change with time. It is accordingly inevitable in practice that the actual measurement value of the distance does not represent the true value in the strict sense but that it includes an error.
According to U.S. Pat. No. 3,321,575 entitled "Television Inspection Apparatus Adapted for Measurement and Comparison Purposes" and issued May 23, 1967, the images of an object of known dimensions and an object of unknown dimensions are taken with a video camera and indicated on display means. Straight traces whose positions are variable are respectively superposed on both the object images indicated, an electric signal which represents the difference of the lengths of both the images along the traces is generated, and a value which is proportional thereto is indicated on an indicator. This technique measures the size of the object of unknown dimensions by directly comparing it with the size of the object of known dimensions. Therefore, when it is applied to the scanning electron microscope, the distance between two points on a specimen can be accurately measured without accompanying the problem of the setting errors of the magnification and the inclination angle of the specimen as previously described.
With such a technique, however, the object of known dimensions or a reference specimen and the object of unknown dimensions or an unknown specimen need to have nearly equal dimensions (lengths). The reason is that, in case where the unknown specimen is much larger than the reference specimen, the dimension (length) measuring precision of the reference specimen lowers, or the measurement becoming difficult. In the converse case, a similar problem occurs as to the unknown specimen. This signifies that, when the sizes of unknown specimens having greatly different dimensions from one another need to be measured, a large number of reference specimens respectively having dimensions approximately equal to those of the unknown specimens must be prepared.